Biotinylated fluorescent ligands are being attached to streptavidin beads, and analyzed by flow cytometry, for the purpose of developing biosensors. This project is described in detail in the year 16-20 NFCR renewal proposal. In brief, the project has three elements 1) to characterize the interaction of biotiylated ligand with strepavidin beads; 2) to detect energy transfer on the bead surface between ligands and receptors; 3) to use the energy transfer between tethered fluorescent ligands and receptors to monitor the presence of soluble ligands. 1) and 2) have been accomplished. Manuscripts for parts 1 and 2 have been accepted for publication (Buranda et al. 1999a; Buranda et al. 1999b). In the upcoming year we will take advantage of the phospholipid on bead strategy used by Nolan to further develop the biosensor. This project complements (Nolan/Swanson) in that it deals with the use of monovalent, reversible ligands rather than crosslinking reagents and detection sc hemes.